Mass/energy transfer processes are known wherein packing particles are placed in a column, or tower into which a gas phase and a liquid phase are introduced in countercurrent fashion. The operating conditions of the system are adjusted so that the bed of packing particles becomes fluidized. Such a system is used, for example, in scrubbing towers for removing undesired substances from gases. This type of system is generally more efficient than a conventional fixed packing system (containing nonfluidized particles and operating in a laminar or semi-turbulent model).
Spherical polypropylene particles have long been used in fluidized beds for gas scrubbing. A disadvantage of these systems is that they cannot sustain large pressure gradients. A high pressure gradient causes channeling and swirling to occur, whereby the particles in the fluidized bed are pushed against each other and wedged to one side in a single static mass of packing by the gas instead of remaining fluidized. Impurities and debris are deposited on these de-fluidized particles. Gas and/or liquid flowing through the particles form channels, and the efficiency of the process is lowered considerably. Also, any remaining fluidized particles need to be frequently replaced due to collisions in the gas swirling through the channels. An improvement on the spherical particles, however, is the use of ellipsoidal particles described in German patent 36 13 151. The use of hollow ellipsoids is considered particularly advantageous.
Application Ser. No. 08/076,481, incorporated by reference herein, is specifically directed to a process for taking advantage of the structure of ellipsoidal particles. In this process, a gas stream is introduced in a column packed with ellipsoid elements to remove heat, gaseous, liquid or particulate matter from a gas stream, or to add to the gas stream (i.e. to remove from the liquid stream) heat, vapor, or moisture. The method includes the steps of:
providing a tower having a bed with fluidizable hollow ellipsoidal packing with a long semi-axis and a short semi-axis;
introducing a liquid stream into the tower at a liquid flow rate L, and countercurrently introducing the gas stream into the tower at a gas flow rate G sufficient to maintain the bed in a fluidized state and a superficial gas velocity v; and
adjusting the volume ratio of the liquid flow to gas flow to cause the ellipsoidal packing to circulate predominantly in the vertical direction relative to the long semi-axis and to maintain a pressure gradient .DELTA.P/H.sub.0 across the depth of the fluidized bed of at least about 1500 Pa/m, and, simultaneously, to satisfy the equation: EQU L/G=K.sub.1 (.DELTA.P/H.sub.0 v)+K.sub.2.
L/G is the volume ratio of liquid to gas; .DELTA.P is the pressure drop across the bed; H.sub.0 is the bed height in a state of rest; v is the superficial gas velocity; and K.sub.1 and K.sub.2 are constants.
By adjusting the parameters set forth in the equation, a fluidized bed can be obtained that does not channel and where pressure gradients can be predicted and set above about 1300-1500 Pa/m. This can achieve significant reduction in tower dimensions (especially height) over traditional fixed packing systems and unstable fluid bed systems (such as those using spheres). Large pressure gradients favor mass and/or energy transfer, which is advantageous in these systems.
There is still a need, however, for a process that allows for the further improved design and/or operation of a heat/energy transfer system using a fluidized packing bed such that the system avoids channeling and optimizes turbulence, for example, by selecting and manipulating not only process operating conditions, but also properties of the packing elements employed such as shape, density, center of gravity or diameter. Such a process is especially useful to upgrade the efficiency of existing mass/transfer installations by choice of packing to be installed therein and/or manipulation of the nature of the fluids and operating parameters of the process.